All crude oil contains impurities which contribute to corrosion, heat exchanger fouling, furnace coking, catalyst deactivation and product degradation in refinery and other processes. These contaminants are broadly classified as salts, bottom sediment and water, solids, and metals. The amounts of these impurities vary depending upon the particular crude. Generally, crude oil salt content ranges between about 3 and 200 pounds per 1000 barrels.
Brines present in crude include predominantly sodium chloride with lesser amounts of magnesium chloride and calcium chloride being present. Chloride salts are the source of highly corrosive HCL which is severely damaging to refinery tower trays, and other equipment. Additionally, carbonate and sulfate salts may be present in the crude in sufficient quantities to promote crude preheat exchanger scaling.
Desalting is, as the name implies, adapted to remove primarily inorganic salts from the crude prior to refining. The desalting step is provided by adding and mixing with the crude a few volume percentages of fresh water to contact the brine and salts present in the crude.
In crude oil desalting, a water-in-oil emulsion is intentionally formed with the water admitted being on the order of about 4 to 10 volume percent based upon the crude oil. Water is added to the crude and mixed intimately to transfer impurities in the crude to the water phase. Separation of the phases occurs due to coalescence of small water droplets into progressively larger droplets and eventually gravity separation of the oil and an underlying water phase occurs.
Demulsification agents are added, usually upstream from the desalter, to help in providing maximum mixing of the oil and water phases in the desalter. Known demulsifying agents include water soluble salts, Twitchell reagents, sulfonated glycerides, sulfonated oils, acetylated castor oils, ethoxylated phenol formaldehyde resins, a variety of polyester materials and many other commercially available compounds.
Desalters are also commonly provided with electrodes to impart an electrical field in the desalter. This serves to polarize the dispersed water molecules. The so formed dipole molecules exert an attractive force between oppositely charged poles with the increased attractive force increasing the speed of water droplet coalescence by from 10 to 100 fold. The water droplets also move quickly in the electrical field, thus promoting random collisions that further enhance coalescence.
Upon separation of the phases from the water-in-oil emulsion, the crude is commonly drawn off the top of the desalter and sent to the fractionator tower in crude units or other refinery processes. The water phase containing water soluble metal salt compounds and the sediment is discharged as effluent.
Emulsion breakers, also called demulsifiers, are usually fed to the crude so as to modify the stabilizer film formed initially at the oil/water interface. These emulsion breakers are surfactants that migrate to the interface allowing droplets of water (or oil) to coalesce more readily. The demulsifiers reduce residence time required for good separation of oil and water.